1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to an improved apparatus for imparting vibrational energy to the earth wherein selected combinations of compressional and shear wave energy can be generated simultaneously.
2. Description of the Prior Art
In applicant's co-pending commonly assigned U.S. Pat. application Ser. No. 826,753, filed on Feb. 6, 1986, entitled "Vibratory Seismic Source for Generating Combined Compressional and Shear Waves" there is described an apparatus for coupling vibratory energy into the earth at a given position on the earth's surface. As described in the subject application a hydraulic vibrator for providing reciprocal vibrations along a vibratory axis is supported above and independently of an earth-contacting base plate. Preferably the vibrator is of a conventional type wherein a reaction mass is slidably mounted on a double rod end piston. Introduction of hydraulic pressure within such vibrator produces significant reciprocation of the piston. Means are provided for variably orienting the vibrator so that such vibratory axis is directly along any predetermined slant path. Further means are provided for lowering the vibrator in the direction of such vibratory axis so that one rod end makes contact with the base plate. Actuation of the vibrator in such contact position then transfers vibrational energy into the base plate by means of which such energy is coupled into the earth. As described in the application the vibrator is preferably mounted upon a rotatable turntable which may, for example, be supported horizontally on the bed of a transport vehicle. The turntable is provided with a central aperture through which the vibrator may be maneuvered in order to contact the base plate. Hydraulic cylinders adjustably interconnected between the turntable and the vibrator vary the inclination of the vibratory axis by rotating the vibrator about an axis parallel to the turntable plane. Rotation of the turntable about its own vertical axis varies the azimuthal direction of such vibrator's axis.